Generally, coupling members, such as bolts, for coupling wheels to a vehicle require high coupling force. Especially, there is a problem in that women, the old or the disabled has difficulty in coupling bolts, etc. for mounting wheels to a vehicle using a conventional manual wrench.
In order to solve such a problem, Korean Patent No. 328512 has proposed a technology for an electric impact wrench, which can couple or disassemble bolts for mounting wheels to a vehicle using the power of an electric motor.
The conventional electric impact wrench will be described in detail with reference to FIG. 1. A shaft 112 of an electric motor 109 is connected to a shaft portion 106a at a side of an inertia wheel 106 rotatably supported by a bearing 113. The inertia wheel 106 has an annular groove formed at the other side thereof. A bearing 114 rotatably supporting a spindle 104 is received in a groove of the inertia wheel 106. The other side of the spindle 104 is supported by a bearing 115.
Further, the inertia wheel 106 has a cut portion 116, which is cut lengthwise along an outer peripheral surface thereof. A blow body 103 is received lengthwise in the cut portion 116. The blow body 103 is installed in the cut portion 116 to pivot upward and downward around a pin 117 in such a manner that the pin 117 is inserted into a pin insertion hole 119 and a pin thru-hole 118 in a state that the pin insertion hole 119 formed in the inertia wheel 106 is coaxial with the pin thru-hole 118 formed in the blow body 103. Specifically, the blow body 103 is subjected to centrifugal force so that a rear portion of the blow body 103 protrudes upward around the pin 117 when the inertia wheel 106 is rotated by a high-speed rotation (about 4000 rpm) of the electric motor 109. Further, the blow body 103 is provided with a weight member 120 at a rear portion thereof so that the rear portion of the blow body 103 is heavier than the front portion, in order to secure a centrifugal movement, and a pair of ball insertion holes 121 is formed at the rear portion of the pin insertion hole 119 formed in the inertia wheel 106 and extends from both sides to the cut portion 116 of the inertia wheel 106. These ball insertion holes 121 are formed at the rear portion of the blow body 103 to coincide with seating grooves 125 with a hemispheric shape when the blow body 103 is installed in the cut portion 116 of the inertia wheel 106.
Ball pressing springs 123 are inserted into the ball insertion holes 121 respectively, in order to press a pair of balls 124 toward the seating grooves 125 formed at both sides of the blow body 103. In the state that the balls 124 and the ball pressing springs 123 are received in the ball insertion holes 121, respectively, screws 122 for adjusting the ball pressing force are coupled to screw portions formed outside of the ball insertion holes 121.
Therefore, if centrifugal force applied to the blow body 103 by the rotation of the inertia wheel 106 is smaller than elastic restitution of the ball pressing springs 123 applied to the seating grooves 125 through the balls 124, the centrifugal movement of the blow body 103 does not occur. However, if the centrifugal force applied to the blow body 103 is larger than the elastic restitution of the ball pressing spring 123 applied to the seating grooves 125, the balls 124 are released from the seating grooves 125 while upraising the rear portion of the blow body 103 using a pin 117 as a rotation point.
Next, as the blow body 103 instantly hits the connector 105 so as to rotate the spindle 104, it is possible to couple or loosen bolts, used for coupling wheels to a vehicle, through a socket mounted on the spindle 104. However, when the spindle 104 does not rotate anymore because the coupling force of the bolts for mounting the wheels is larger than the rotation force of the spindle 104, or the coupling of the bolts for mounting the wheels is completed, the rotation of the spindle 104 stops.
On the other hand, a return spring 126 is provided on the rear portion of the blow body 103 in order to pull the rear portion of the blow body 103 to an initial position (i.e. a position before the centrifugal force is applied to the rear portion of the blow body 103). Both ends of the return spring 126 are connected to protrusions 127 formed on both sides of the blow body 103 respectively, so that the return spring 126 has a loop shape. The loop-shaped return spring 126 sits on a shaft 106a of the inertia wheel 106.
Therefore, when the coupling force of the bolts for mounting wheels is larger than the rotation force of the spindle 104, or the bolts for mounting the wheels are completely coupled, as described above, the centrifugal force applied to the blow body 103 can be removed. When the centrifugal force applied to the blow body 103 is smaller than the elastic restitution of the return spring 126 for returning the blow body 103 to an initial position, the blow body 103 is returned to its initial position by the elastic restitution of the return spring 126. Thus, as the coupling of the blow body 103 to the connector 105 is released during the return of the blow body 103 to the initial position, it is possible to remove the load applied to the electric motor 109.